Typical low-intensity prescribed burn in a bur oak savanna.

Typical flames from oak leaves in a savanna burn. Note the curled form of the leaves, which promotes the "carry" of the fire from one leaf to the next. The temperature is hot enough to top-kill woody shrubs and trees such as maple, elm, and walnut, but does not affect the thick bark of the oak trees.

These characteristics make savanna burns possible. It is likely that oaks have evolved these leaf characteristics as part of their evolution as a fire-dependent species. The temperatures reached in burns is very high, generally more than 300-500 F, but occur only for very brief periods of time. Mineral soil is a very poor conductor of heat, so that there are few if any changes in the character of the soil as a result of a burn. The black seen after a burn is from the ash created by burned littler. The ashes formed from the burn quickly mineralize.

Principles of prescribed burns

The fire triangle presents the three components necessary for initiation of a fire: fuel, oxygen, and heat. Fuel, the most important factor, consists of organic materials that are capable of being oxidized. Oxygen is essential for combustion, since burning is an oxidation process. However, combustion will not begin unless heat is applied. Heat raises the fuel to the combustion temperature. At this point, usually around 300-500 F, a fire exists. Once combustion has begun, the combustion process creates further heat, making it possible for the fire to propagate itself. Propagation continues until all combustible fuel is used up, or until conditions change.

The flames of a burning fuel arise from the combustion of gases which are driven off the burning fuel. Thus, when one sees visible flames one is seeing an event resulting from the formation of combustible gases by the burning fuel itself.

The combustibility of the fuel will depend, among other things, on its moisture content, its physical structure, and its chemical content. Wet fuel will not burn, or will burn only poorly. The drier the fuel, the more flammable it will be. Physical structure refers to the sizes and shapes of the fuel particles. Large logs burn slower than small logs, which burn slower than twigs, which burn slower than forbs, which burn slower than grass. The principle chemical components of wildland fuels are cellulose and lignin, but some plants also produce resins or other chemicals that are highly flammable (not common in Midwest plants).

Dead plant material is more flammable than living plants. Living plants consist predominantly of water (60-120% or more moisture content), but once the plant is cut from its roots, or dies, it begins to lose moisture (curing). The rate at which moisture is lost will depend on the environmental conditions as well as upon the physical structure of the plant material. Under a given weather condition, grasses lose moisture much more rapidly than shrubs or trees.

Types of fuel

Here we are discussing dead fuels. Because of their high moisture contents, living fuels are rarely a consideration in savanna burns.

Grass Dead grasses vary in their flammability, but in general they are the most flammable fuels in prescribed burns. Tall grass species such as Indian grass and big bluestem burn hotter and with higher flames than short grass species such as side oats grama and little bluestem. The flame height of tall grass is higher and the rate of movement of the flaming front much more rapid than that of short grass. Grasses tend to lose moisture more rapidly than woody plant material (become more quickly flammable), but they also gain moisture more rapidly.

Forbs Although in arid regions of the United States many flammable forbs exist, in the Midwest forbs are relatively fire proof. A forbs-dominated site usually burns poorly, and does not carry a fire well.

Shrubs Because of their physical structure, brush and dead shrubs are less combustible than grasses, although small diameter brush that is very dry can burn rapidly.

Trees Because of their physical structure, dead trees are less combustible than shrubs, although piles of dead logs that are very dry (for instance; have been lying on the ground for some months or years) can be highly combustible. The flammability of oak leaves was discussed above.

Time lags of fuels

A wet dead fuel will start to dry when the humidity of the air is less than 100%. The rate at which the fuel dries depends, among other things, upon its size. Small diameter fuels dry more rapidly than large diameter fuels. The time lag of a fuel is expressed as the time it takes for a fuel particle to reach 63% of its equilibrium moisture content. Time lag is important in deciding when after major rain events a burn can be scheduled. Time lag is also important because the flames themselves will dry out the fuel, and if the time lag is short, fire may be possible even if the fuel is moderately damp.

Four time-lag classes are recognized:

One-hour time lag fuels are those fuels that are less than ¼ inch in diameter. These fuels are the most important for carrying surface fires, and if present their behavior usually controls fire behavior. One-hour fuels include oak leaves, grassy fuels, and small twigs. Grasses are the most important one-hour fuels because they are standing vertically in the air, where they can equilibrate rapidly.

Ten-hour fuels include small branches and woody stems, from ¼ to 1 inch diameter. They resist drying and have a higher heat capacity than one-hour fuels and hence often do not burn up in low-intensity surface fires. Under low humidity conditions, however, ten-hour fuels can carry hot fires and help ignite the larger-sized fuels.

100-hour fuels include larger downed woody material, with diameters from 1-3 inches. These fuels take longer to dry and hence combust slowly or not at all under most conditions. However, if 100-hour fuels do ignite, the large fuel load means that they can continue to burn for long periods of time.

1000-hour fuels include large downed branches, logs, and tree stumps, with diameters greater than 3 inches. They burn only under prolonged dry conditions or when preheated by adjacent burning fuels. These 1000-hour fuels can act as firebreaks during savanna burns, preventing the spread of the fire across the forest floor. They can also act as fire shadows, creating unburned areas on the upwind or uphill side of the fire front. If these fuels do ignite, they can often burn for days, creating problems in mop-up.

The various categories just mentioned refer to the time lag before sufficient moisture has dissipated so that combustion can be initiated. However, it takes different amounts of time for fuels in these various categories to combust even when the moisture content is low enough for combustion. This is because the igniting flame must heat the fuel up to the combustion temperature, and finer fuels heat up faster than coarser fuels. Thus, a lighted match may be sufficient to bring dry grass to the ignition temperature, whereas the same match would have little effect on a log. This is a matter of compactness of the fuel as well as its heat capacity. The log has a high heat capacity and takes a long time to reach its combustion temperature.

Downed bramble branches illustrating a 10-hour fuel. These brambles had been cut the previous winter and were well cured (dried).

The categorization of fuels by time lag implies that they are all subject to the same environmental conditions. However, this is almost never the case. Environmental factors that affect the rate of drying include air temperature, relative humidity, and wind speed. For instance, a ten-hour fuel subjected to a 20 mph wind at a temperature of 80 F and relative humidity of 20% will dry exceedingly rapidly and will become flammable long before the ten-hour time has been reached.

Flaming and smoldering

There are two kinds of burning. Flaming occurs when the heat of combustion breaks down the fuel into volatile combustible products (hydrogen, carbon monoxide, hydrocarbons) which burn quickly. Flaming produces the least amount of smoke, and dissipates quickly as the fine fuel is used up. Flaming provides the visible evidence that a fire exists.

Smoldering occurs when oxygen is limited, such as when fuels are tightly packed (for instance, a log pile), in wet fuels, or in fuels that have already been charred. Smoldering produces the most amount of smoke. Smoldering combustion may continue for long periods of time on the forest floor, increasing the possibility of hazard if wind picks up and carries embers away.

Example of a smoldering tree. This bur oak, still living, was hollow and caught on fire from a ground layer burn. The fire was extinguished with a high-pressure water spray.

It is possible for smoldering combustion to convert into flaming combustion, or vice versa. If smoldering combustion causes the breakdown of the fuel into smaller particles, the fuel may then undergo transition to a state in which it bursts into flame. Also, often simply adding oxygen (air) to a smoldering fuel is sufficient to cause it to burst into flames.

On the other hand, flaming combustion can convert into smoldering combustion if the most flammable fuel elements have been used up, leaving behind only the heavier, more tightly packed fuel elements.

An understanding of the events leading to transition from flaming to smoldering or vice versa is essential, especially in the mop-up phase of savanna burns.

In some savanna burns, fire begins as smoldering and after sufficient heat has been generated bursts into flaming combustion.


Factors influencing moisture content of dead fuels

Dead fuels dry by evaporation into the atmosphere. Whether or not a site will burn will depend strongly on fuel moisture, which will in turn depend on its previous history, especially over the last 24-48 hours. Primary weather factors influencing fuel moisture are sun, wind, rainfall, relative humidity, and air temperature. Several nice sunny, windy, rainless days with low humidity and warm temperatures make an ideal lead-in to a burn.

Slope and aspect also influence fuel drying. Aspect refers to the direction the terrain is pointing (N, S, E, or W), and slope refers to the angle of the terrain, both in relation to the sun. In Midwestern latitudes, steep, south-facing slopes receive more direct sunlight than flat land and hence dry more rapidly.

Factors influencing flammability of the fuel in an oak savanna

Flammability can refer to the speed with which a fuel ignites, the intensity of the heat it produces, and the rate at which fire spreads. The leaves of oaks burn hotter than those of any other Midwest forest trees. They carry a fire effectively because they do not lie flat on the ground, but curl up, so that they equilibrate more rapidly with the atmosphere. Also, the curl makes it more likely that fire will pass from one leaf to the next. (See photo above)

Because of the variability in the oak savanna habitat, burns of several types can be anticipated. Burns in very open areas (one tree per acre, for instance) where the dominant fuel is tall grass, are equivalent to prairie burns. Grass is the ideal fuel to “carry” a fire, and this is what makes a prairie burn relatively easy to carry out.

However, grasses vary in flammability. Grasses such as Indian grass, big and little bluestem, and side oats grama burn extremely hot, and carry a fire very well. Depending on wind, humidity, slope, and aspect, flame heights can be 5 feet high or higher.

These prairie grasses will not succeed in shadier areas (30-50% canopy), where cool season (savanna) grasses often flourish. Cool-season grasses, ryes and bromes, burn cooler and do not carry a fire nearly as well.

The principal fuel in shadier areas of an oak savanna are oak leaves, which are vital to the burn. Oak leaves are admirably suited for fire (see above) and it is undoubtedly because of their nature that the oak savanna habitat exists.

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